Does adding PlotRange->All help? (I cannot test at the moment). I think the problem is that show takes the options (including the plot range) is taken from the first argument, and that's too small to see the circle. Another thing you could do is to put the large circle first (because AFAICS it doesn't intersect with the RegionPlot anyway).
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celtschkMar 24 '12 at 16:42

If you posted actual working code showing the problem, the chances would be better that anyone understands what your problem is.
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celtschkMar 24 '12 at 17:29

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